Conventionally, a joint device for an artificial leg has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 11-345. The joint device is applied to an ankle joint for connecting between a foot member of an artificial leg and an under-knee member of the same. The foot member of the artificial leg has an upper end portion thereof formed with a through hole extending laterally. On the other hand, the under-knee member has a lower end thereof bifurcated into two arms to form a bracket with each arm having a hole formed therethrough. In this joint device, the upper end portion of the foot member is fitted between the arms of the bracket, and a shaft is fitted through the holes of the bracket and the through hole of the foot member. Thus, the foot member and the under-knee member are connected to each other such that they can perform pivotal motion with respect to each other about a horizontal axis, only in the front-rear direction, depending on a reaction force generated when the foot member of the artificial leg lands.
Further, conventionally, varieties of artificial leg-joint devices have also been proposed which are used as a knee joint for connecting between an above-knee member and an under-knee member. Among these, there is a type having the above-knee member and the under-knee member connected to each other in a manner pivotally movable with respect to each other about a horizontal axis such that the two members are locked when the knee joint is stretched, and unlocked when the user of the artificial leg triggers motion for bending the knee from the knee stretched state, whereby the under-knee member is swung forward by centrifugal force.
However, the above joint device is not capable of moving the foot member and the under-knee member actively, but the foot member and the under-knee leg portion just passively perform pivotal motion with respect to each other depending on a reaction force generated e.g. by landing of the foot member. Therefore, differently from a non-handicapped person, the user cannot freely perform “toe-up motion” (see FIG. 27C) for moving a leg forward or “kicking motion” (see FIG. 27B) for kicking the ground. If the user fails to perform toe-up motion, the toe of the foot member is easily caught e.g. at an uneven spot on a road, which can cause a fall of the user in the worst case. On the other hand, inability to perform kicking motion cannot lead to a fall of the user, but when the user is walking on an unpaved road, no dust is raised, which reveals the use of the artificial leg. This inflicts such a big pain on the user of the artificial leg as non-handicapped people cannot imagine. Another type of joint device has also been proposed which is capable of performing active motion by using an electric motor. However, this joint device suffers from other problems, such as a short duration of power supply and an increased weight.
Further, although the above-mentioned conventional joint device has the function of swinging the under-knee member forward by utilizing a centrifugal force, triggering motion for causing the swinging motion is troublesome, which makes it difficult to perform “knee bending/stretching motion” (see FIGS. 27B to 27D). As a result, actually, in many cases, the user does not use the function of swinging the under-knee member forward, but performs “swinging-out motion” for moving the artificial leg forward by swinging the leg outward in its stretched state. This swinging-out motion immediately reveals the use of the artificial leg, which is most painful to the user of the artificial leg. As can be understood from the above, users of artificial legs earnestly desire that they will be freed from the need of performing swinging-out motion and be able to easily perform knee bending/stretching motion.